If you initiate a patent infringement suit against FB, then you are back to your original rights under the BSD grant. Facebook could potentially counterclaim with their own patents covering React at that point — if they have any and if they are actually enforceable, which is not known.

But this is actually a better situation than obtains under the BSD license alone, where you could be preemptively sued at any point by a patentholder without the precondition of having to initiate a suit against FB first.

The fundamental point is understanding that the patent grant doesn’t change or sabotage any rights under the BSD. It’s an additional grant of rights that can terminate under certain conditions which to date remain completely hypothetical.